it's perfectly legal to charge for content you created.
You'd think so, but then,
[quote]* Zombie_dust
- Zombie_torn
- Zombie_dust2
- Zombie_boxes
- Zombie_aztec
- Zombie_aztec_micro
- Zombie_training
- Zombie_cspace1
- Zombie_base[/quote]
These appear to just be popular and Official CS maps decompiled and changed to work for CS Zombiemod or whatever it's called. So it's not really their work.
EDIT:
All of the following maps listed are the property of the PCL and the OGC are protected by copyright:
Yes, because it's possible to copyright something you stole from another author. Basically, whoever is charging for this thinks they have a right to make people pay them for another person's work, just because they can decompile a map, add different entities for the Terrorist spawns, and recompile it again. Most likely without VIS. Or full RAD.